/**
*      author: Y_Yao
*      created: 2025-02-09 12:03:02
**/

#include<bits/stdc++.h>

#define int LL
using namespace std;
using LL = long long;
const LL LLINF = 1e18;
const int INF = 0x3f3f3f3f;
const int N = 100 + 7;
int n;
unordered_map<int,double> m1[N];

void solve()
{
    cin >> n;
    for(int i = 1;i <= n;i++)
    {
        int k;
        cin >> k;
        for(int j = 1;j <= k;j++)
        {
            int x;
            cin >> x;
            m1[i][x]++;
        }

        for(auto& e : m1[i])
        {
            e.second = e.second / k;
        }
    }

    double ans = 0;
    for(int i = 1;i <= n;i++)
    {
        for(int j = i + 1;j <= n;j++)
        {
            double ret = 0;
            for(auto& e : m1[i])
            {
                if(m1[j].count(e.first))
                {
                    ret += e.second * m1[j][e.first];
                }
            }
            ans = max(ans,ret);
        }
    }

    printf("%.15lf",ans);
}

signed main()
{
    // ios::sync_with_stdio(false);
    // cin.tie(0);

    int t = 1;
    //cin >> t;
    while(t--)
    {
        solve();
    }
    return 0;
}